1. Field of the Invention
This invention relates to the processing of electronic article surveillance (EAS) tag responses, and more particularly to a system and method of processing that removes stationary EAS tag responses signals from EAS tag detection.
2. Description of the Related Art
An acoustomagnetic or magnetomechanical EAS system interrogates an EAS tag by transmitting an electromagnetic burst at a resonance frequency of the tag. The tag responds with an acoustomagnetic or magnetomechanical response frequency that is detectable by the EAS system receiver. At the end of the transmitter burst, the system detects the exponentially decaying response of the tag. The tag signal amplitude rapidly decays to ambient noise levels, so the time interval in which the tag signal can be detected is limited. U.S. Pat. No. 4,510,489 discloses such an EAS system, one embodiment of which is sold under the trademark ULTRAMAX by Sensormatic Electronics Corporation, Boca Raton, Fla.
In the above-described process, the transmitter burst signal does not end abruptly but instead decays exponentially because of transmitter circuit reactance. The tag signal cannot be detected until this circuit “ringdown” has essentially disappeared. Therefore, the time period during which the tag signal can be detected is reduced. This is a particular problem because the circuit ringdown occurs while the tag signal is at its largest. An additional detection problem occurs when a tag is stationary in the fringe of the detection zone. As the ambient noise varies during the day, a tag that is far enough away from the receiver to not be detected most of the day may be detected when the noise levels decrease below a certain level. This is a common problem in the retail environment where a display rack of tagged merchandise is located near a store entrance where the EAS detection or interrogation zone is located. It is desired that the system ignore the stationary item(s) and yet detect a tag moving through the detection zone.
Previous solutions for the circuit ringdown problem have been to hold detection until the ringdown signal is over, and by trying to control circuit reactance to minimize the ringdown effect. Waiting until the ringdown period is over sacrifices detection because the tag response signal is highest immediately after the transmit burst. Items placed near the antenna (such as display racks), which vary by location, affect circuit reactance and make the circuit reactance difficult to control. In addition, transmitter power amplifier design relies on the circuit Q being fairly high, which limits how low the reactance can be adjusted.
A previous attempt at a solution to the stationary tag problem, which was relatively unsuccessful, involved storing the time domain tag response signal in a memory buffer, which is replica of the tag signal, and subtracting the replica signal from the received tag signal before attempting tag detection. However, the system needed to be able to detect that the tag signal is not moving before it adds the signal to the replica. In addition, it needed to be able to detect when the tag had been removed, otherwise the subtraction of the replica signal from the input resulted in an “anti-replica” which caused a system alarm that continues until the system stops subtracting the replica.